Method for determining a background count rate in liquid scintillation counting

ABSTRACT

The present invention provides a method for determining a background count rate in liquid scintillation counting. The method comprises measuring an external standard spectrum (201, 202) of a sample, determining, from the external standard spectrum, an external standard count rate within an energy window (203), determining, based on the external standard count rate within the energy window, a background reference parameter, and determining, based on the background reference parameter, the background count rate from a background reference curve (301).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for determining a backgroundcount rate in liquid scintillation counting according to the preamble ofthe appended independent claim.

BACKGROUND OF THE INVENTION

Liquid scintillation counting is a widely used technique to measure theradioactivity of a sample. The measurement technique is applicable toall forms of nuclear decay emissions, such as alpha and beta particles,electron capture and gamma ray emitting radionuclides. In liquidscintillation counting, the sample is placed in direct contact with aliquid scintillation medium to which the sample is dissolved orsuspended. The liquid scintillation medium contains a mixture ofchemicals that can absorb, for example, the energy of alpha or betaparticles and convert the absorbed energy to light.

The emitted light is measured using one or more photodetectors.Registering light pulses during the time of measurement enables torepresent the number of light pulses as a function their energy (pulseheight). This energy spectrum is indicative of the amount of activityassociated with the radionuclides in the measured sample. The frequencyof light pulses (typically presented in counts per minute) correspondsto the absolute activity in the sample (decays per minute) thateventually corresponds to the number of radionuclides in the sample.

In liquid scintillation counting, the measurement of the sample consistsof net counts obtained from actual radioactive decays and differentsources of background. The background sources are, among other, thermalrandom coincidence pulses from photodetectors, random luminescencecoincidence pulses and pulses generated by ambient radioactivityincluding gamma radiation from the surrounding material and cosmicradiation. Thermal background can be reduced by coincidence counting andby cooling the photodetectors. Lead shielding can be used to protect thesample and the photodetectors from ambient radioactivity.

FIG. 1 illustrates an example of a liquid scintillation counting systemthat employs a coincidence counting technique to reduce background. Thesystem comprises a scintillation vial 101 for holding a radioactivecocktail and two diametrically opposed photomultiplier tubes (PMTs) 102and 103 for detecting and amplifying the photons emitted from theradioactive cocktail and converting them into electrical pulses. Theoutputs of the PMTs 102 and 103 are connected to a coincidence pulsedetector 104, which compares the electrical signals received from thePMTs 102 and 103. The coincidence pulse detector 104 produces an outputsignal only when the two electrical signals occur together. By onlyaccepting coincidence pulses, the system can discriminate single pulsesthat are caused by thermal background and single photon noise, such asluminescence. The output signal of the coincidence pulse detector 104 isfed to a multichannel analyser (MCA) 105, which produces an energyspectrum representing the amount and characteristics of theradioactivity in the sample.

In many liquid scintillation counting applications, it is necessary tobe able to distinguish the net counts of the sample from the background.For this purpose, a specific background sample that is chemically likethe (unknown) sample is prepared and measured. The measured counts fromthe background sample are subtracted from the gross counts of the sampleto obtain the net counts of the sample. A problem associated with thistechnique is that the preparation of the background sample can be verydifficult or even impossible, due to various chemical and/or technicalreasons.

OBJECTIVES OF THE INVENTION

It is the main objective of the present invention to reduce or eveneliminate the prior art problems presented above.

It is an objective of the present invention to provide a method fordetermining a background count rate in liquid scintillation counting. Inmore detail, it is an objective of the invention to provide a method fordetermining a background count rate without preparing and measuring aspecific background sample that is chemically like the sample undermeasurement. It is also an objective of the present invention to providea method for determining a background count rate in a quick and accuratemanner.

In order to realise the above-mentioned objectives, the method accordingto the invention is characterised by what is presented in thecharacterising portion of the appended independent claim. Advantageousembodiments of the invention are described in the dependent claims.

DESCRIPTION OF THE INVENTION

A method according to the invention for determining a background countrate in liquid scintillation counting comprises measuring an externalstandard spectrum of a sample, determining, from the external standardspectrum, an external standard count rate within an energy window,determining, based on the external standard count rate within the energywindow, a background reference parameter, and determining, based on thebackground reference parameter, the background count rate from abackground reference curve.

The method according to the invention enables to determine thebackground count rate for a sample without preparing and measuring aspecific background sample that is chemically like the sample undermeasurement. In the method according to the invention, the backgroundcount rate is determined from the predefined background reference curvethat represents the background count rate as a function of thebackground reference parameter. The background count rate is typicallypresented in counts per minute (CPM).

In the first step of the method according to the invention, an externalstandard spectrum of a sample is measured. The external standardspectrum of the sample is achieved by using an external gamma radiationsource in a liquid scintillation counting system. The external standardspectrum represents an external standard count rate as a function ofenergy (pulse height). The external standard count rate is typicallypresented in counts per minute. The gamma radiation produces a widespectrum of energies of Compton electrons via the Compton effect. Thegamma radiation source can be, for example, Eu-152 isotope or Ba-133.

In the second step of the method according to the invention, an externalstandard count rate within an energy window (counting window) isdetermined from the external standard spectrum. Preferably, a lowerlimit of the energy window is between 1 and 10 keV and an upper limit ofthe energy window is between 30 and 75 keV.

In the third step of the method according to the invention, a backgroundreference parameter is determined based on the external standard countrate within the energy window. The background reference parameter isdetermined by applying a function to the external standard count rate.In its simplest form, the background reference parameter corresponds tothe external standard count rate. The applied function can be continuousor splined. The function can be dependent on the amount of quenching, sothat different functions are used for samples with low quench and highquench. Quenching is defined as an irreversible absorption of decayenergy during the energy transfer from the decaying particle to thephotodetector. Quenching shifts the energy spectrum towards lowerenergies and results in the reduction of the counting efficiency.

In the fourth step of the method according to the invention, thebackground count rate is determined from a background reference curvebased on the background reference parameter. This is achieved by findingthe associated background count rate for the value of the backgroundreference parameter from the curve.

The background reference curve is predefined by using a set ofbackground samples with variable quench. This set can be made, forexample, by using a pure liquid scintillation cocktail as a base andadding different amounts of a quench agent to obtain the backgroundsamples. For each background sample, the background reference parameteris determined as well as the actual count rate is measured. Thebackground reference curve can be established by plotting all thedatapoints to a count rate versus background reference parameter graphand by fitting a curve to these datapoints.

The method according to the invention can be applied in a liquidscintillation counting system to determine the background count rate fora sample. In the system, the sample is placed with a liquidscintillation medium in a scintillation vial. The liquid scintillationmedium contains a mixture of chemicals that can absorb, for example, theenergy of alpha or beta particles and convert the absorbed energy tolight, which is measured using one or more photodetectors. Theregistered light pulses during the time of measurement can be presentedas an energy spectrum that is a plot representing the number orfrequency of the light pulses as a function of their energy (pulseheight). From this energy spectrum, a gross sample count rate within anenergy window can be determined. The liquid scintillation countingsystem comprises an external gamma radiation source. By irradiating thescintillation vial with gamma radiation, an external standard spectrumof the sample can be measured. By using the method according to theinvention, the background count rate can be determined from the externalstandard spectrum of the sample. The net sample count rate can then beobtained by subtracting the background count rate from the gross samplecount rate.

The background reference curve can be stored into a memory of the liquidscintillation counting system and used to determine the background countrate for different samples. The background reference curve can beupdated if the energy window changes, the external standard changes, orthe external standard activity has dropped due to decay ofradioactivity.

An advantage of the method according to the invention is that abackground count rate can be determined without preparing and measuringa specific background sample that is chemically like the sample undermeasurement. Another advantage of the method according to the inventionis that a background count rate can be determined in a quick andaccurate manner.

According to an embodiment of the invention the background referencecurve is generated by using a plurality of background samples havingdifferent quenches and performing the following steps for eachbackground sample: measuring an external standard spectrum of thebackground sample, determining, from the external standard spectrum, anexternal standard count rate within the energy window, determining,based on the external standard count rate within the energy window, abackground reference parameter, measuring an energy spectrum of thebackground sample, and determining, from the energy spectrum, abackground sample count rate within the energy window; plotting thebackground sample count rates against the background referenceparameters, and fitting a curve to the datapoints to obtain thebackground reference curve.

The external standard spectrum of the background sample is achieved byusing an external gamma radiation source in liquid scintillationcounting. The external standard spectrum represents an external standardcount rate as a function of energy (pulse height). The energy spectrumof the background sample is achieved without using an external gammaradiation source. The energy spectrum represents a background samplecount rate as a function of energy (pulse height). In determining theexternal standard count rate and the background sample count rate withinthe energy window, the same energy window is used like in the case ofmeasuring the sample. The background reference parameter is determinedby applying a function to the external standard count rate within theenergy window. In its simplest form, the background reference parametercorresponds to the external standard count rate.

According to an embodiment of the invention the background referenceparameter is calculated by an equation:Ref=ECounts*ScalingConstant,

where ECounts is the external standard count rate within the energywindow and ScalingConstant is a scaling constant, which is determinedwhen the background reference curve is created. The scaling constant is,for example, subject to the external standard activity level, whichdecays over time requiring re-determination of the curve. This equationcan be used for calculating the background reference parameters for thesample and the plurality of the background samples.

According to an embodiment of the invention the method comprisesdetermining, from the external standard spectrum, an external standardquench parameter, wherein the background reference parameter isdetermined based on the external standard count rate within the energywindow and the external standard quench parameter. This embodimentconcerns both the sample and the plurality of the background samples.The background reference parameter is determined by applying a functionto the external standard count rate within the energy window and theexternal standard quench parameter.

According to an embodiment of the invention the background referencecurve is generated by using a plurality of background samples havingdifferent quenches and performing the following steps for eachbackground sample: measuring an external standard spectrum of thebackground sample, determining, from the external standard spectrum, anexternal standard count rate within the energy window and an externalstandard quench parameter, determining, based on the external standardcount rate within the energy window and the external standard quenchparameter, a background reference parameter, measuring an energyspectrum of the background sample, and determining, from the energyspectrum, a background sample count rate within the energy window;plotting the background sample count rates against the backgroundreference parameters, and fitting a curve to the datapoints to obtainthe background reference curve.

The external standard spectrum of the background sample is achieved byusing an external gamma radiation source in liquid scintillationcounting. The external standard spectrum represents an external standardcount rate as a function of energy (pulse height). The energy spectrumof the background sample is achieved without using an external gammaradiation source. The energy spectrum represents a background samplecount rate as a function of energy (pulse height). In determining theexternal standard count rate and the background sample count rate withinthe energy window, the same energy window is used like in the case ofmeasuring the sample. The background reference parameter is determinedby applying a function to the external standard count rate within theenergy window and the external standard quench parameter.

According to an embodiment of the invention the background referenceparameter is calculated by an equation:Ref=ECounts*ScalingConstant1, when QP>=QP _(threshold), orRef=ECounts*ScalingConstant1+(QP _(threshold) −QP)*ScalingConstant2,when QP<QP _(threshold),

where ECounts is the external standard count rate within the energywindow, QP is the external standard quench parameter, ScalingConstant1is a first scaling constant, ScalingConstant2 is a second scalingconstant and QP_(threshold) is an external standard quench parameterthreshold. ScalingConstant1, ScalingConstant2 and QP_(threshold) aredetermined when the background reference curve is created. The scalingconstants are, for example, subject to the external standard activitylevel, which decays over time requiring re-determination of the curve.This equation can be used for calculating the background referenceparameters for the sample and the plurality of the background samples.

According to an embodiment of the invention the background samplecontains a liquid scintillation cocktail and a quench agent. The quenchagent is used to vary quench and it can be, for example, nitromethane(chemical quench) or Sudan I (colour quench). Preferably, differentbackground samples contain different amounts of the quench agent.

According to an embodiment of the invention the number of backgroundsamples is at least 6. This enables to provide an enough accurateestimate of the background reference curve. The number of backgroundsamples can be, for example, 6-10, 10-20 or 20-50.

According to an embodiment of the invention the external standard quenchparameter is the spectral endpoint of the external standard spectrum.

According to an embodiment of the invention a lower limit of the energywindow is between 1 and 10 keV and an upper limit of the energy windowis between 30 and 75 keV.

The present invention also relates to a method for determining a netsample count rate in liquid scintillation counting. The method comprisesmeasuring an energy spectrum of a sample, determining, from the energyspectrum, a gross sample count rate within an energy window, determininga background count rate according to a method according to theinvention, and subtracting the background count rate from the grosssample count rate to obtain the net sample count rate.

The determined net sample count rate includes an error due to chemicaland colour quenching. Chemical quenching prevents the energy transferfrom a decay particle to the scintillator and thus it reduces theinitial photon output. Colour quenching prevents the generated photonsfrom reaching the photodetector.

Different techniques are known to overcome quench issues. For example,there are different quench correction methods that are based ondetermination of the shift of the spectrum. The spectrum shift isrelative to a change in a counting efficiency.

A plurality of standard samples with known radioactivity and variablequench can be used to generate a quench curve that can be used todetermine a counting efficiency for the sample and hence to correct themeasured raw counts to absolute radioactive decays in the sample. Thequench curve can be generated in such a manner that for each standardsample a quench parameter is measured, and the efficiency is calculatedby dividing the observed radioactivity with the known radioactivity. Thequench curve can be established by plotting all the datapoints to thecounting efficiency versus the quench parameter graph and by fitting acurve to these datapoints.

The counting efficiency for the sample is determined by firstdetermining, from the external standard spectrum of the sample, anexternal standard quench parameter, and then determining, based on theexternal standard quench parameter, the counting efficiency from thequench curve. The absolute activity in the sample (decays per minute)that eventually corresponds to the number of radionuclides in the sampleis determined by dividing the net sample count rate by the countingefficiency.

According to an embodiment of the invention the sample containscarbon-14. Carbon-14, or radiocarbon, is a radioactive isotope of carbonwith an atomic nucleus containing 6 protons and 8 neutrons.

Example: Determination of Biological Content in Diesel Oil

Biological content in diesel oil can vary from 0% to 100%. Due to taxsubsidiaries for bio-based products and regulatory reasons, it isimportant to be able to detect biological content in oil-based products,such as diesel oil. Chemical analysis (mass spectrometry, etc.) canidentify molecular mixture in diesel but it cannot confirm whether themolecules are of biological origin. However, radiocarbon content in thesample will give net biological content of the carbon present. Cosmicradiation converts nitrogen atoms into radioactive carbon (C-14) in theupper atmosphere with constant flux. Radiocarbon bounds then to CO2molecules, which are absorbed into plants in photosynthesis. This willgive biological products a constant level of C-14 radioactivity thatequals to about 13.5 DPM (decays per minute) in gram of total carbon. Ashalf-life of C-14 is about 5700 years, there is no C-14 activity infossil fuels since the radioactivity has decayed over the millions ofyears the oil has been captured underground. Thus, any oil-based samplehas an internal fingerprint of biological content with C-14 activity.Zero DPM per gram of carbon equals with no biological origin while 13.5DPM equals with 100% biological origin. And proportionally 1.35 DPMequals with 10% of biological origin.

Liquid scintillation counting is a practical method to count theradioactivity of C-14 in these samples. There are some challenges thatthe methodology needs to address though. One is the inherently lowradioactivity of these types of samples. Basic methods to overcome thelow radioactivity is to use sensitive instruments and long countingtimes to obtain statistically meaningful number of total counts.Counting times of around 6 hours per sample are commonly used. Inaddition, maximizing the sample size yields to higher sensitivity. Asthe measured radioactivity is between 0 and 13.5 DPM per gram of totalcarbon, it is beneficial to measure a large amount of the sample inorder to obtain a high number of counts from the radioactive carbon. Inorder to maximize the sample size, the preferred method is to mix thesample (diesel in this case) with a liquid scintillation cocktaildirectly. With e.g. 20 ml vial size, 50%-50% sample to cocktail ratio wecan measure 10 ml of diesel in a single measurement. The amount of 10 mlof diesel weights 8 grams and has about 7 grams of total carbon (0.86gC/gFUEL). With 7 grams of carbon the sample DPM would vary from 0 DPMto 95 DPM for fully fossil and fully biological diesel, respectively.With other sample preparation methods, the total carbon content will belower. E.g. in oxidizing, a maximum of 1 gram of diesel could be treatedthat would yield 8 times lower counts, which would be difficult tocount.

With direct counting there are two key challenges: highly variablequench and difficulty to make a blank background sample with exactlysimilar chemical composition with the unknown sample. Random dieselsample contains a mixture of oil molecules that vary from diesel brandto brand as well as different colours are added to diesel for processmonitoring purposes that end up into the product. Variable quench (i.e.variable counting efficiency) can be managed using standard liquidscintillation counting quench correction methods. The background sampleremains an issue, which can be solved with the present invention.

By utilising the method according to the invention, the biologicalcontent n diesel oil can be determined as follows:

-   -   1. Establish a background reference curve using a set of        quenched background samples that do not contain C-14        radioactivity. The set can, for example, contain 10 background        samples. This set can be made, for example, by using liquid        scintillation cocktail and quench agents. Preferably, both        chemical quenching agents and colour quenching agents are used.        The background reference curve is established as follows:        -   a. Process each background sample (A . . . J) as follows:            -   i. Perform two measurements: one with external standard                and one normal beta counting. For background sample A                with external standard, record Quench Parameter (QP A)                and counts within a counting window (Counts A). In                normal counting mode, record observed counts that would                then be background of measured sample (Bkg A).            -   ii. Calculate background reference value (ref A) using                Counts A. The simplest form of this calculation would be                “ref A=Counts A”. This works in some cases. However,                sometimes a splined function is preferred. Measurements                can show that different calculation is needed for highly                quenched samples, thus the function should be relative                to QP A as well i.e. “ref A=f(Counts A, QP A).        -   b. Draw background reference curve by plotting each ref A .            . . ref J against Bkg A . . . Bkg J.    -   2. For the unknown diesel sample (let's call it sample S)        -   a. Perform two measurements: one with external standard and            one normal beta counting. From the external standard            spectrum, record QP S and Counts S. Using normal beta            counting measure sample(CPM) i.e. the C-14 counts from the            sample.        -   b. Calculate background reference value: ref S=f(Counts S,            QP S)        -   c. Read background counts Bkg(CPM) from the background            reference curve using ref S.        -   d. Determine from the quench curve the counting efficiency            c_eff using sample(QP)=QP S    -   3. Calculate net sample counts:        net_sample(CPM)=sample(CPM)−Bkg(CPM)    -   4. Calculate absolute sample activity:        Sample(DPM)=net_sample(CPM)/c_eff    -   5. Calculate DPM per gram of carbon from the Sample(DPM) and        determine how much biological origin there is in the sample.

The exemplary embodiments of the invention presented in this text arenot interpreted to pose limitations to the applicability of the appendedclaims. The verb “to comprise” is used in this text as an openlimitation that does not exclude the existence of also unrecitedfeatures. The features recited in the dependent claims are mutuallyfreely combinable unless otherwise explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a liquid scintillation counting systemthat employs a coincidence counting technique.

FIG. 2 illustrates two examples of an external standard spectrum.

FIG. 3 illustrates an example of a background reference curve, and

FIG. 4 illustrates an example of a quench curve.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 has already been described in connection with the background ofthe invention. Embodiments of the invention will now be described withreference to FIGS. 2 to 4 .

FIG. 2 illustrates two examples of an external standard spectrum. InFIG. 2 , there are shown external standard spectra measured from twodifferent samples, namely sample A and sample B. The external standardspectra of the samples A and B are indicated with reference numbers 201and 202, respectively. The external standard spectra 201 and 202represent an external standard count rate as a function of energy (pulseheight). The external standard spectra 201 and 202 were achieved byusing an external gamma radiation source in a liquid scintillationcounting system. The gamma radiation produced a wide spectrum ofenergies of Compton electrons via the Compton effect.

From each external standard spectrum 201 and 202, an external standardcount rate within an energy window 203 (counts A, counts B) and anexternal standard quench parameter (QP A, QP B) can be determined. Theexternal standard quench parameter is the spectral endpoint of theexternal standard spectrum 201, 202. The external standard count ratewithin the energy window 203 can be used in determining a backgroundcount rate for the sample from a background reference curve. Theexternal standard quench parameter can be used in determining thecounting efficiency for the sample from a quench curve. In some cases,the external standard quench parameter can also be used in determiningthe background count rate for the sample.

FIG. 3 illustrates an example of a background reference curve. Thebackground reference curve 301 represents a background count rate as afunction of a background reference parameter. The background referencecurve 301 was created by using a set of blank background samples withvariable quench. For each background sample, the background referenceparameter was determined as well as the actual count rate was measured.The background reference curve 301 was established by plotting all thedatapoints to the count rate versus the background reference parametergraph and by fitting a curve to these datapoints.

The background count rates for the samples A and B can be determinedfrom the background reference curve 301 as follows. First, thebackground reference parameters (Ref A, Ref B) are calculated byapplying a function to the external standard count rates within theenergy window (counts A, counts B). Then, the background count rates(Bkg A, Bkg B) are determined from the background reference curve 301 byfinding the associated background count rates for the values of thebackground reference parameter.

FIG. 4 illustrates an example of a quench curve. The quench curve 401represents a counting efficiency as a function of a quench parameter.The quench curve 401 was created by using a plurality of standardsamples with known radioactivity and variable quench. For each standardsample, the quench parameter was measured, and the counting efficiencywas calculated by dividing the observed radioactivity with the knownradioactivity. The quench curve 401 was established by plotting all thedatapoints to the counting efficiency versus the quench parameter graphand by fitting a curve to these datapoints. The quench curve 401 can beused to determine a counting efficiency for the samples A and B. Thecounting efficiency (Eff A, Eff B) for the samples A and B is determinedfrom the quench curve 401, based on the external standard quenchparameters (QP A, QP B). The absolute activity in the samples A and Bthat eventually corresponds to the number of radionuclides can bedetermined by dividing the net sample count rate by the countingefficiency.

Only advantageous exemplary embodiments of the invention are describedin the figures. It is clear to a person skilled in the art that theinvention is not restricted only to the examples presented above, butthe invention may vary within the limits of the claims presentedhereafter. Some possible embodiments of the invention are described inthe dependent claims, and they are not to be considered to restrict thescope of protection of the invention as such.

The invention claimed is:
 1. A method for determining a background countrate in liquid scintillation counting, comprising: measuring an externalstandard spectrum of a sample placed in direct contact with a liquidscintillation medium, wherein the external standard spectrum representsan external standard count rate as a function of energy, thus allowingaccounting for both color and chemical quench; determining, from theexternal standard spectrum, the external standard count rate within anenergy window; determining, from the external standard spectrum, anexternal standard quench parameter; determining, based on the externalstandard count rate within the energy window and the external standardquench parameter, a background reference parameter; and determining,based on the background reference parameter, the background count ratefrom a background reference curve.
 2. The method according to claim 1,wherein the background reference curve is generated by: using aplurality of background samples having different quenches and performingthe following steps for each background sample: measuring an externalstandard spectrum of the background sample, determining, from theexternal standard spectrum, an external standard count rate within theenergy window, determining, based on the external standard count ratewithin the energy window, a background reference parameter, measuring anenergy spectrum of the background sample, and determining, from theenergy spectrum, a background sample count rate within the energywindow; plotting the background sample count rates against thebackground reference parameters, and fitting a curve to the datapointsto obtain the background reference curve.
 3. The method according toclaim 1, wherein the background reference parameter is calculated by anequation:Ref=ECounts*ScalingConstant, where ECounts is the external standardcount rate within the energy window and ScalingConstant is a scalingconstant.
 4. The method according to claim 1, wherein the backgroundreference curve is generated by: using a plurality of background sampleshaving different quenches and performing the following steps for eachbackground sample: measuring an external standard spectrum of thebackground sample, determining, from the external standard spectrum, anexternal standard count rate within the energy window and an externalstandard quench parameter, determining, based on the external standardcount rate within the energy window and the external standard quenchparameter, a background reference parameter, measuring an energyspectrum of the background sample, and determining, from the energyspectrum, a background sample count rate within the energy window;plotting the background sample count rates against the backgroundreference parameters, and fitting a curve to the datapoints to obtainthe background reference curve.
 5. The method according to claim 1,wherein the background reference parameter is calculated by an equation:Ref=ECounts*ScalingConstant1, when QP>=QP _(threshold), orRef=ECounts*ScalingConstant1+(QP _(threshold) −QP)*ScalingConstant2,when QP<QP _(threshold), where ECounts is the external standard countrate within the energy window, QP is the external standard quenchparameter, ScalingConstant1 is a first scaling constant,ScalingConstant2 is a second scaling constant and QP_(threshold) is anexternal standard quench parameter threshold.
 6. The method according toclaim 2, wherein the background sample contains a liquid scintillationcocktail and a quench agent.
 7. The method according to claim 2, whereinthe number of background samples is at least
 6. 8. The method accordingto claim 1, wherein the external standard quench parameter is thespectral endpoint of the external standard spectrum.
 9. The methodaccording to claim 1, wherein a lower limit of the energy window is 1and 10 keV and an upper limit of the energy window is between 30 and 75keV.
 10. A method for determining a net count rate in liquidscintillation counting, comprising: measuring an energy spectrum of asample, determining, from the energy spectrum, a gross sample count ratewithin an energy window, wherein the method comprises: determining abackground count rate according to claim 1, and subtracting thebackground count rate from the gross sample count rate to obtain the netsample count rate.
 11. The method according to claim 1, wherein thesample contains carbon-14.